It is known that it is desirable to produce elastomeric polymers capable of exhibiting reduced hysteresis when properly compounded with other ingredients such as reinforcing agents and then vulcanized. Such elastomers, when fabricated into components for constructing articles such as tires, vibration isolators, power belts, and the like, will manifest properties of increased rebound, decreased rolling resistance and less heat-build up when subjected to mechanical stress during normal use.
The hysteresis of an elastomer refers to the difference between the energy applied to deform an article made from the elastomer and the energy released as the elastomer returns to its initial, undeformed state. In pneumatic tires, lowered hysteresis properties are associated with reduced rolling resistance and heat build-up during operation of the tire. These properties, in turn, result in lowered fuel consumption of vehicles using such tires and prolonged tire life. In such contexts, the property of lowered hysteresis of compounded, vulcanizable elastomer compositions is particularly significant. Examples of such compounded elastomer systems are known to the art and are comprised of at least one elastomer (that is, a natural or synthetic polymer exhibiting elastomer properties, such as a rubber), a reinforcing filler agent (such as finely divided carbon black, thermal black, or mineral fillers such as clay and the like) and a vulcanizing system such as sulfur-containing vulcanizing (that is, curing) system.
Various synthetic strategies have been developed to provide elastomers with molecular structures exhibiting reduced hysteresis energy losses. One technique is to produce elastomers of very high molecular weight. In such high molecular weight systems, the number of free, uncrosslinked molecular chain-ends per given weight in the vulcanizates made from them are reduced. Since the presence of free, unbound chain ends are believed to be a significant factor in hysteretic energy loss because they cannot participate in elastic recovery processes, their reduction leads to a desirable reduction in hysteretic energy loss.
Another technique is to prepare elastomer molecules with end groups capable of interacting with the reinforcing fillers such as carbon black present in compounded elastomer compositions. Again, such interaction reduces the number of free end groups believed to contribute to hysteretic losses. Such interactive end groups include those derived from tin or other metal reagents as well as those derived from polar organic reagents such amines, amides, esters, imines, imides, ketones and various combinations of such groups. For example, commonly assigned U.S. Pat. application Ser. No. 07/703,533 now U.S. Pat. No. 5,151,469 describes elastomers with end-groups derived by reacting an anionically prepared polymer having a reactive lithium site with about an equivalent of an alkyl or aryl sulfoxide compound to provide an elastomer with polar end groups. This application does not disclose or suggest reaction of polymer with a vinyl sulfoxide which, under the anionic conditions used in this invention, occurs through the vinyl functionality. Thus, the capping reactions occuring in the present invention are inherently different than those occuring with alkyl or aryl sulfoxides.
The present invention is directed to vinyl sulfoxide-capped elastomers which, when used to prepare a compounded and vulcanized by known rubber processing techniques, provide vulcanized elastomers which exhibit desirable low hysteresis characteristics. These (vinyl sulfoxide)-capped elastomers can be made by reacting lithium - terminated elastomer intermediates with more than one equivalent of at least one vinyl sulfoxide. The (vinyl sulfoxide)-capped elastomers thereby produced contain short blocks of poly (vinyl sulfoxide) having a degree of polymerization of about 1 or more. Mixtures of (vinyl sulfoxide)-capped elastomers with the uncapped elastomer derived from the lithium terminated elastomer are also useful. For convenience, these mixtures can be described as having a degree of polymerization less than 1.0, say 0.5. In a preferred aspect of the invention, the vinyl sulfoxide-capped elastomers have blocks with degrees of polymerization greater than 1.0, usually greater than two. These are poly (vinyl sulfoxide)-capped elastomers where the degree of polymerization of the vinyl sulfoxide block is 2 or more, the elastomer molecules have polar end-groups derived from two or more molecules of vinyl sulfoxide.